EXHAUST AFTER TREATMENT SYSTEM WITH A PASSIVE NOx ADSORBER AND A HEATABLE SCR CATALYTIC CONVERTER

ABSTRACT

The invention relates to an exhaust after treatment system with a passive NOx adsorber and a heatable SCR catalytic converter. The exhaust gas after treatment system according to the invention for an internal combustion engine comprises an exhaust pipe, a passive NOx adsorber arranged in the exhaust pipe, an injector arranged in the exhaust pipe, a mixer arranged in the exhaust pipe, an SCR catalytic converter arranged in the exhaust pipe and a heating device arranged in the exhaust pipe, wherein the heating device is set up in such a way that at exhaust gas temperatures below a start-up temperature of the SCR catalytic converter, the temperature in the SCR catalytic converter can be brought to a temperature above the start-up temperature within a period of time by means of the heating device.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority of German Patent Application No. DE 102018129683.5 filed Nov. 26, 2019.

FIELD OF THE INVENTION

The disclosure relates to an exhaust after treatment system with a passive NO_(x) adsorber and a heatable SCR catalytic converter.

BACKGROUND OF THE INVENTION

An exhaust after treatment system for an internal combustion engine is known from U.S. Pat. No. 8,661,790B2 that includes an exhaust pipe, an upstream SCR catalytic converter, an electrically heated catalytic converter, a downstream SCR catalytic converter and a control unit. The oxidation catalytic converter can be heated by the electrically heated catalytic converter. A NO_(x) adsorber for adsorbing NO_(x) emissions below a temperature threshold and for desorbing NO_(x) emissions above a temperature threshold is arranged in the electrically heated catalytic converter and/or the oxidation catalytic converter. The electrically heated catalytic converter is switched on when the operating temperature of the downstream SCR catalytic converter is above the start-up temperature. As a result, the temperature in the NO_(x) adsorber increases and the desorption of NO_(x) emissions is initiated.

The exhaust after treatment system according to the disclosure for an internal combustion engine comprises an exhaust pipe, a passive NO_(x) adsorber arranged in the exhaust pipe, an injector arranged in the exhaust pipe, a mixer arranged in the exhaust pipe, an SCR catalytic converter arranged in the exhaust pipe and a heating device arranged in the exhaust pipe. The heating device is set up in such a way that at exhaust gas temperatures below a start-up temperature of the SCR catalytic converter, the temperature in the SCR catalytic converter can be brought to a temperature above the start-up temperature within a period of time by means of the heating device.

The combination of a passive NO_(x) adsorber (PNA) and an SCR (selective catalytic reduction) catalytic converter is suitable for reducing emissions of nitrogen oxides (NO_(x)). The idea is that the PNA adsorbs NO_(x) emissions during the cold start and warm-up phases of the combustion engine. As soon as the SCR catalytic converter reaches the start-up temperature, the PNA can be desorbed. For the combined use of a PNA and an SCR catalytic converter, the temperature properties of the PNA must be taken into account, as this affects the possibility of having an empty PNA at the end of engine operation. Thus, low adsorption stability of the NO_(x) can cause very early NO_(x) desorption, if for example strong acceleration of the internal combustion engine takes place in cold start conditions or in the warm-up phase. This results in high temperatures in the PNA with continued low temperatures in the SCR catalytic converter. The NO_(x), that is desorbed in the low-temperature PNA due to the high temperatures cannot therefore be converted in the SCR catalytic converter.

The exhaust after treatment system according to the disclosure combines the idea of an electrically heated SCR catalytic converter with a PNA. This allows the conversion of NO_(x) emissions in the SCR catalytic converter by heating the SCR catalytic converter within a period of time, even if very early NO_(x) desorption in the passive NO_(x) adsorber takes place at exhaust gas temperatures below the start-up temperature of the SCR catalytic converter. A time span of less than 90 seconds is advantageous, especially advantageous is less than 60 seconds.

SUMMARY OF THE INVENTION

An advantageous embodiment of the disclosure provides that the heating device is designed as an electrical heat source. An electrical heat source is very easy to control, as switching operations involve hardly any delays. Due to the separate power supply, for example by a battery, it can also be used largely independently of the operating point of the internal combustion engine.

A further advantageous embodiment of the disclosure provides that the NO_(x) adsorber is implemented as a low-temperature adsorber. This enables an empty PNA at the end of engine operation as a result of purely passive NO_(x) desorption, so that a large part of the storage capacity of the PNA is available at the next engine start.

Preferably, the exhaust after treatment system according to the disclosure comprises a control unit, which is set up to perform the following steps: switching on the heating device, determining the temperature in the SCR catalytic converter, switching off the heating device if the temperature in the SCR catalytic converter has exceeded a temperature threshold. The temperature in the SCR catalytic converter can be determined by means of a sensor, a model or a characteristic field. The sensor can either directly measure the temperature in the SCR catalytic converter or is positioned in the exhaust pipe to measure the exhaust gas temperature. Several sensors can also be used, so that, for example, a temperature upstream of the SCR catalytic converter and a temperature downstream of the SCR catalytic converter are determined. The determination of the temperature in the SCR catalytic converter can be supplemented by a model in order to be able to determine the temperature in the SCR catalytic converter more precisely. This is particularly useful if the temperature in the SCR catalytic converter is measured indirectly by a measurement of an exhaust gas temperature. The temperature threshold is preferably in a range of ±20° C., particularly preferably in a range of ±10° C. of the start-up temperature of the SCR catalytic converter. The starting temperature in a catalytic converter is understood to be the temperature at which the catalytic reactions start to take place. Experience has shown that this is in the range of 200-300° C. for SCR catalytic converters.

Preferably, the control unit is set up to first determine the temperature in the SCR catalytic converter and to switch on the heating device if the temperature in the SCR catalytic converter falls below a further temperature threshold. This has the advantage that the heating device is only switched on when this is necessary. Particularly preferably, the temperature in the SCR catalytic converter is determined continuously, so that the heating device can be switched on or off depending on the state of the exhaust gas after treatment system. The further temperature threshold is lower than the temperature threshold.

The dependent claims describe further advantageous embodiments of the disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred embodiments are explained in more detail using the following figures. In the figures

FIG. 1 shows an exhaust after treatment system according to the disclosure, wherein the heating device is attached directly to the SCR catalytic converter,

FIG. 2 shows an exhaust after treatment system according to the disclosure, wherein the heating device is attached directly to the passive NO_(x) adsorber,

FIG. 3 shows a method for heating an SCR catalytic converter of an exhaust after treatment system that is carried out by a control unit.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows an exhaust aftertreatment system according to the disclosure for an internal combustion engine. The exhaust aftertreatment system comprises an exhaust pipe 1, a passive NO_(x) adsorber 2 arranged in the exhaust pipe 1, an injector 3 arranged in the exhaust pipe, a mixer arranged in the exhaust pipe 4, an SCR catalytic converter 5 arranged in the exhaust pipe and a heating device 6 arranged in the exhaust pipe. The exhaust pipe 1 is fluidically connected to the outlet of the combustion engine and the exhaust gas of the combustion engine flows through the exhaust pipe 1. The exhaust gas reaches the passive NO_(x) adsorber 2 via the exhaust pipe 1. The passive NO_(x) adsorber 2 is designed to adsorb NO_(x) emissions contained in the exhaust gas. When the storage capacity of the passive NO_(x) adsorber 2 is exhausted and/or a sufficiently high temperature is reached, the NO_(x) emissions are desorbed. An injector 3 and a mixer 4 are arranged in the exhaust pipe 1 between the passive NO_(x) adsorber 2 and the SCR catalytic converter 5. A reducing agent, for example a urea solution, is added to the exhaust gas via the injector 3. The mixer 4 allows homogenization of the exhaust gas and the introduced urea solution. The NO_(x) emissions desorbed in the passive NO_(x) adsorber 2 are reduced in the SCR catalytic converter 5. A heating device 6 is attached directly to the SCR catalytic converter 5. This heating device 6 is set up in such a way that at exhaust gas temperatures below a start-up temperature of the SCR catalytic converter 5, the temperature in the SCR catalytic converter 5 can be brought to a temperature above the start-up temperature within a period of time by means of the heating device 6. This ensures that the SCR catalytic converter 5 is ready for operation when the storage capacity of the passive NO_(x) adsorber 2 is exhausted, or that the time interval between desorption of the passive NO_(x) adsorber 2 and reaching the start-up temperature of the SCR catalytic converter 5 is at least minimized.

FIG. 2 shows another exemplary embodiment of the exhaust aftertreatment system, wherein the heating device 6 is attached directly to the passive NO_(x) adsorber 2. This allows the use of heatable passive NO_(x) adsorbers that are available on the market, so that costs can be reduced. Alternatively, the heating device 6 can be arranged upstream of the SCR catalytic converter 5 between the passive NO_(x) adsorber 2 and the mixer 4 or upstream of the SCR catalytic converter 5 between the mixer 4 and the SCR catalytic converter 5. This flexibility in the possible arrangement of the heating device 6 allows advantageous adaptation to different exhaust aftertreatment systems.

In the exemplary embodiments shown of the exhaust aftertreatment system, the heating device 6 is implemented as an electrical heat source. This has the advantage that for some exhaust aftertreatment components such as passive NO_(x) adsorbers 2, combined heating systems and NO_(x) adsorbers 2 are available, which can reduce costs. In addition, electrical heat sources allow precise control as they have short delay times. Furthermore, electrical heat sources can be supplied by an external power supply, such as a battery, whereby they can be operated largely independently of the operating state of the internal combustion engine.

The NO_(x) adsorber 2 is implemented here as a low-temperature adsorber. Low-temperature adsorbers are understood to be passive NO_(x) adsorbers that desorb NO_(x) emissions at relatively low temperatures, for example between 150 and 200° C. The use of a low-temperature adsorber is advantageous, since an empty passive NO_(x) adsorber 2 at the end of the engine operation is enabled by a purely passive NO_(x) desorption.

In the exemplary embodiments shown, the exhaust aftertreatment system includes a control unit. This control unit is set up to perform the steps shown in FIG. 3. In the first step S10 the heating device 6 is switched on, in the second step S20 the temperature in the SCR catalytic converter 5 is determined and in the third step S30 switching off the heating device 6 is initiated if the temperature in the SCR catalytic converter 5 has exceeded a temperature threshold.

The control unit is set up to switch on the heating device 6 if it is recognized that a desorption of the passive NO_(x) adsorber 2 is required at a point in time and the temperature in the SCR catalytic converter 5 will not exceed a temperature threshold by that time without heating. This enables adaptive operation of the heating device 6 by switching on the heating device 6 depending on the load state of the passive NO_(x) adsorber 2 and depending on the thermal state of the SCR catalytic converter 5. This has the advantage that the SCR catalytic converter 5 has highly advantageous conversion rates when desorption of the passive NO_(x) adsorber 2 is performed.

Alternatively, the control unit can be set up to switch on the heating device 6 immediately when the engine is started. Immediately when the engine is started means within 10 seconds after the engine is started, preferably within 5 seconds after the engine is started, particularly preferably within 2 seconds after the engine is started. This form of control is easier to implement but is also less accurate.

In this exemplary embodiment, the control unit is set up to regulate the power of the heating device 6 depending on the load state of the passive NO_(x) adsorber 2 and the temperature in the SCR catalytic converter 5. This achieves the technical advantage that not only is switching on and off of the heating device 6 used, but a power level of the heating device 6 that is adapted to the operating condition of the combustion engine and/or the thermal condition of the exhaust aftertreatment system is used. Thus, the power of the heating device 6 can be reduced, for example if the difference between the temperature determined in the SCR catalytic converter 5 and a temperature threshold allows the temperature threshold value to at least be reached within an advantageous period of time even with lower power. For this regulation of the heating device 6, models and/or characteristics are particularly suitable that make it possible to describe at least parts of the exhaust aftertreatment system predictively and a heating device 6 that allows an adjustment of the power output that goes beyond mere switching on and off.

Here the control unit is set up to model the load state of the passive NO_(x) adsorber 2 and the temperature in the SCR catalytic converter 5. In alternative embodiments that are not shown, a state of the passive NO_(x) adsorber and/or the SCR catalytic converter 5 is modeled and/or measured based on a temperature, a level of a conversion behavior and/or a storage behavior. The heating device 6 is then controlled on the basis of this modeled and/or measured state. 

1. Exhaust after treatment system for an internal combustion engine, comprising an exhaust pipe, a passive NO_(x) adsorber arranged in the exhaust pipe, an injector arranged in the exhaust pipe, a mixer arranged in the exhaust pipe, an SCR catalytic converter arranged in the exhaust pipe and a heating device arranged in the exhaust pipe, wherein the heating device is set up that so at exhaust gas temperatures below a start-up temperature of the SCR catalytic converter, the temperature in the SCR catalytic converter can be brought to a temperature above the start-up temperature within a period of time by the heating device.
 2. Apparatus according to claim 1, wherein the heating device is attached directly to the SCR catalytic converter.
 3. Apparatus according to claim 1, wherein the heating device is arranged upstream of the SCR catalytic converter between the passive NO_(x) adsorber and the mixer.
 4. Apparatus according to claim 1, wherein the heating device is arranged upstream of the SCR catalytic converter between the mixer and the SCR catalytic converter.
 5. Apparatus according to claim 1, wherein the heating device as an electric heat source.
 6. Apparatus according to claim 1, wherein the NO_(x) adsorber is a low-temperature adsorber.
 7. Apparatus according to claim 1, including a control unit having a structure performing the following steps: switching on the heating device, determining the temperature in the SCR catalytic converter, switching off the heating device if the temperature in the SCR catalytic converter has exceeded a temperature threshold.
 8. Apparatus according to claim 7, wherein the control unit switches on the heating device immediately when the engine is started.
 9. Apparatus according to claim 7, wherein the control unit switches on the heating device if it is recognized that desorption of the NO_(x) adsorber is required at a point in time and that the temperature in the SCR catalytic converter will not exceed a temperature threshold by that point in time without heating.
 10. Apparatus according to any one of claim 7, wherein the control unit regulates the power of the heating device depending on the load state of the NO_(x) adsorber and the temperature in the SCR catalytic converter. 